Large-capacity optical communication systems have been promoted along with the spread of the Internet. For example, for a trunk line system, an optical transmitter and an optical receiver capable of transmitting signals with 40 Gbit/s or more per wavelength are studied.
When a bit rate per wavelength is increased, an optical signal to noise ratio (OSNR) tolerance is degraded and the deterioration of signal quality due to waveform distortion caused by the chromatic distribution, polarization mode distribution, non-linear effect of the transmission line, and the like increases. Therefore, a digital coherent receiver with high OSNR tolerance and high waveform distortion tolerance has been focused on recently.
In the digital coherent receiving method, optical amplitude information and phase information is extracted from a received signal and the received signal is demodulated by a digital signal processing circuit. In the digital coherent receiving method, since OSNR tolerance is improved by coherent reception and waveform distortion is compensated for by a digital signal processing circuit, even an optical communication system with 40 Gbit/s or more obtains a high reliability. A method for receiving optical QPSK signals by coherent detection is described, for example, by D. Ly-Gagnon et al., “Coherent Detection of Optical Quadrature Phase-Shift Keying Signals With Carrier Phase Estimation”, IEEE, Journal of Lightwave Technology, vol. 24, No. 1, pp. 12-21, January 2006.
FIG. 1 illustrates a configuration of a digital coherent optical receiver. In FIG. 1, a 90-degree optical hybrid circuit includes first and second input ports and first and second output ports. An optical signal and local oscillator light are input to the first and second input ports, respectively. The local oscillator light is generated by a laser light source provided for a receiver. The optical signal and the local oscillator light are mixed and output from the first output port. The 90-degree optical hybrid circuit includes a 90-degree phase shift element. The optical signal and the local oscillator light whose phase is shifted by 90 degrees are mixed and are output from the second output port. One set of optical signals output from the first and second output ports are converted into electrical signals by respective photo detectors. Then the electrical signals are converted into digital signals by respective A/D converters and are given to the digital signal processing circuit.
One set of digital signals given to the digital signal processing circuit indicate a real part component and a imaginary part component in the case where an input optical signal is expressed by a complex electric field. Then, the digital signal processing circuit demodulates the input optical signal using this set of digital signals.
When the 90-degree phase shift element has a phase error (quadrature error or quadrature angle error) in the digital coherent optical receiver having the above configuration, crosstalk is caused between the real part signal and the imaginary part signal. In this case, the demodulation performance of the digital coherent optical receiver deteriorates. In particular, in a modulation method whose spectral usage efficiency is high (that is, MPSK, MQAM and the like in which the number of transmitted bits per symbol is large), since quality deterioration is sensitive to the phase error of the 90-degree phase shift element, a technique for compensating for the phase error is required.
As a method for compensating for the phase error of the 90-degree optical hybrid circuit, the following procedure is proposed. A method is provided for correcting a quadrature angle error that exists in the coherent receiver hardware of a dual-polarization optical transport system. The receiver hardware that causes the quadrature angle error is a 90 degree optical hybrid mixing device. The method involves generating an estimate of the quadrature angle error and compensating for the quadrature angle error by multiplying the first and second detected baseband signals by coefficients that are a function of the estimate of the quadrature angle error. (for example, U.S. Pat. No. 6,917,031)
However, when the above method is realized by a hardware circuit, the circuit scale increases. When the calculation speed of a processor is taken into account, it is difficult to realize the above method by software in a receiver for receiving Gbit/s signals.